Main areas of the microwave analog phase shifters application are antenna arrays, such as focusing control for radar and communication antennas and array beam steering, and use as microwave wireless power transport (WPT) chargers. Furthermore, the microwave analog phase shifters may be used as phase modulators for modulating a phase of an input radio frequency (RF) signal.
General requirements for the microwave analog phase shifters are 360° phase shift, low insertion loss, low transmission coefficient ripple, compact size, printed circuit board (PCB) integrability as well as low cost.
Tunable elements which are traditionally used in the microwave analog phase shifters may include semiconductors and dielectric varactors, and piezoelectric capacitors. The most common design of the analog phase shifters with tunable capacitances includes a 3-dB hybrid coupler (a 2-branch line coupler, a rat-race coupler, a Lange coupler, or the like) with a tunable reflective load. The tunable reflective load usually includes two tunable capacitors that are tuned simultaneously by applying a varying direct current (DC) voltage. As a result, the losses occur that are caused by an inner resonance within a reflective load structure. The resonance losses lead to the high attenuation of an RF output signal and parasitic amplitude modulation. Such phenomena may degrade system performance, that is, may reduce array gain (if used in antenna arrays), or may distort a signal spectrum (if used in a phase modulator).
Known in the art is a phase shifter disclosed in U.S. Pat. No. 7,969,359 B2. US' 359 B2 teaches a phase shifter including: a hybrid coupler which is ground shielded and includes differential coplanar strip lines placed one on top of the other using different metal layers so that signal coupling occurs vertically; and reflective terminations which are connected to the hybrid coupler such that when the hybrid coupler is connected to the reflective terminations, a phase shifter is formed, the reflective terminations each including a parallel LC circuit. Disadvantages of the known solution are high resonance losses, high ripple, and high sensitivity of a phase response to voltage changes.
In addition, known in the art is a phase shifter disclosed in U.S. Pat. No. 6,710,679 B2 (U.S. '679). U.S. '679 teaches a 360° analog dielectric varactor phase shifter, including a 180° analog rat-race ring phase shifter, and a 180° digital switch line phase shifter, and the digital phase shifter includes first and second microstrip lines connected to each other through capacitors. One of the above microstrip lines serves as a 180° phase shift line and another microstrip line serves as a reference lines. Disadvantages of the known solutions are very bulky design and high resonance losses.
Therefore, in designing a phase shifter, there is a demand for a solution having a simplified design and a reduced size with minimal losses to maintain the same amplitude ripple and a 360° phase range.